
*{
    margin: 0;
    padding: 0;
}
.add-box{
    position: relative;
    width: 100px;
    height: 100px;
    background-color: red;
}
.add{
    position: absolute;
    left: 0;
    top: 0;
    z-index: -10;
    width: 100%;
    height: 100%;
}